A typical computing power supply for laptops and related devices such as tablets uses a combination of Lithium Ion (Li-Ion) batteries, usually arranged in groups of two cells in series that produces a maximum voltage of approximately 10V. Such a relatively high power supply voltage is unsuitable for modern integrated circuits so mobile personal computers (PCs) conventionally include a buck converter to regulate the battery power supply voltage from the series-connected batteries to an internal power supply voltage such as 1V for powering the integrated circuits within the device.
A single stage multi-phase buck converter would require high voltage components to step down from such a relatively high battery power supply voltage to the relatively low internal power supply voltage. The use of such high voltage components demands substantial die space to achieve suitable drain to source resistance and also leads to higher gate drive losses and voltage-current overlap switching losses for the power switches. Thus, single stage multi-phase buck converters are not very efficient in applications in which the output voltage is substantially stepped down from, for example, around 10V to 1V.
To improve the efficiency, two-stage multi-phase DC/DC power converters have been used. A first stage comprises a step-down switched capacitor (charge pump) stage that converts the input voltage from the batteries into an intermediate charge pump output voltage. A second stage multi-phase buck converter converts the intermediate charge pump output voltage into the regulated internal power supply voltage. But the efficiency of such multi-stage power converters at low output load remains a problem.
Accordingly, there is a need in the art for an improved multi-stage DC/DC power converter with efficient operation at low output loads.